1. Field of Invention
The present invention relates to a mixer. More particularly, the present invention relates to a sub-harmonic mixer having LC-folded cascode.
2. Description of Related Art
A direct-conversion receiver (or homodyne receiver) uses one time down conversion operation in its structure for directly converting a radio frequency (RF) signal into a base band signal, so it is also called zero-IF receiver. The frequency of the poly-phase local oscillation (LO) signal produced by such receiver is very close to the frequency of the RF signal, thus, image noise interference can be avoided, and no image rejection filter is required to be disposed before mixing the RF signal and the poly-phase LO signal because of the disappearance of image noise. Accordingly, compared to other receiver structures such as a superheterodyne receiver, a direct-conversion receiver has the advantages such as simple structure and single chip, and is being adopted more and more in today's transceivers.
Even though the direct-conversion receiver has the foregoing advantages, the structure thereof still has some disadvantages. For example, DC offset is one of the problems thereof. The production of DC offset is mainly because that the isolation between the input terminal (here referred to as input terminal of RF signal) of the mixer and the low noise amplifier (LNA) and the input terminal of the mixer for receiving the poly-phase LO signal is not infinite, thus, when the poly-phase LO signal appears at the input terminal of RF signal due to feedthrough effect, the poly-phase LO signal will be self-mixed with the original poly-phase LO signal and further the DC offset will be produced. In addition, the problem of even-order distortion should be noted too because the direct-conversion receiver does not have an image rejection filter before mixing the signals, so that the even-order distortion produced by non-linear circuit may also be directly transmitted to the output of the mixer along with the disturbing signal around the RF signal through feedthrough effect, so that the RF signal to be received originally may be further affected.
To resolve the aforementioned problems, the direct-conversion receiver adopts the conventional sub-harmonic mixer (SHM) as shown in FIG. 1 for providing ideal isolation to the input terminals of the poly-phase LO signal and the RF signal. The conventional SHM is derived from Gilbert mixer as shown in FIG. 2. Referring to FIG. 1 and FIG. 2, wherein the poly-phase LO signal includes local oscillation signals LO1_0°˜LO1_270° which respectively have phase shifts of 0°, 90°, 180°, and 270°, and the poly-phase local oscillation signal LO2 includes local oscillation signals LO2_0° and LO2_180° which respectively have phase shifts 0° and 180°. If each of the N-type transistors MN9˜MN12 in Gilbert mixer is replaced with two N-type transistors connected in parallel, and the frequency provided to the poly-phase LO signal LO2 of the Gilbert mixer is reduced to 0.5 time of the original frequency, so as to form the poly-phase LO signal LO1 received by NMOS transistors connected in parallel and accordingly the conventional SHM in FIG. 1. For example, replace MN9 with MN1 and MN2, and the frequencies with phase difference 180° of LO signals LO1_0° and LO1_180° received by MN1 and MN2 is 2 times of that of the LO signal LO2_0° received by MN9. Accordingly, the direct-conversion receiver with conventional SHM can operate the frequency of the poly-phase LO signal at 0.5 times of the frequency of the RF signal, and can also maintain the ideal isolation of Gilbert mixer.
However, in the actual monolithic process, the isolation provided by the conventional SHM is always affected due to unsymmetrical circuit caused by mismatching components. Thus, how to improve the isolation of the input terminal of a conventional SHM by using original circuit structure, so as to reduce the leakage signal of the poly-phase LO signal and even-order distortion caused by disturbing signal at the input terminal of the RF signal, has become the biggest challenge in the application of direct-conversion receiver.